In probability theory, the Wick product is a particular way of defining an adjusted product of a set of random variables. In the lowest order product the adjustment corresponds to subtracting off the mean value, to leave a result whose mean is zero. For the higher order products the adjustment involves subtracting off lower order (ordinary) products of the random variables, in a symmetric way, again leaving a result whose mean is zero. The Wick product is a polynomial function of the random variables, their expected values, and expected values of their products.
The definition of the Wick product immediately leads to the Wick power of a single random variable and this allows analogues of other functions of random variables to be defined on the basis of replacing the ordinary powers in a power-series expansions by the Wick powers. The Wick powers of commonly-seen random variables can be expressed in terms of special functions such as Bernoulli polynomials or Hermite polynomials.
The Wick product is named after physicist Gian-Carlo Wick, cf. Wick's theorem.
Definition
Assume that X1, ..., Xk are random variables with finite moments. The Wick product
![{\displaystyle \langle X_{1},\dots ,X_{k}\rangle \,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/bccc07610150c04aaf5f2147858a26c4e2e73028)
is a sort of product defined recursively as follows:[citation needed]
![{\displaystyle \langle \rangle =1\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/af8d1af4a8a2bd7e669af3ac12cc6964c6fa8db8)
(i.e. the empty product—the product of no random variables at all—is 1). For k ≥ 1, we impose the requirement
![{\displaystyle {\partial \langle X_{1},\dots ,X_{k}\rangle \over \partial X_{i}}=\langle X_{1},\dots ,X_{i-1},{\widehat {X}}_{i},X_{i+1},\dots ,X_{k}\rangle ,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e3b207ff4fc5b6a3932bb4cc56f74433765421bf)
where
means that Xi is absent, together with the constraint that the average is zero,
![{\displaystyle \operatorname {E} \langle X_{1},\dots ,X_{k}\rangle =0.\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/3193232e8415af21cc17522e7c2f0f542be90f20)
Equivalently, the Wick product can be defined by writing the monomial
as a "Wick polynomial":
,
where
denotes the Wick product
if
. This is easily seen to satisfy the inductive definition.
Examples
It follows that
![{\displaystyle \langle X\rangle =X-\operatorname {E} X,\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ce459b21803a373a3ef76cb3737600f6e001c2bc)
![{\displaystyle \langle X,Y\rangle =XY-\operatorname {E} Y\cdot X-\operatorname {E} X\cdot Y+2(\operatorname {E} X)(\operatorname {E} Y)-\operatorname {E} (XY),\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/f4bc88ee2c26d47ac4c0c110c98e2fe0aa8ce842)
![{\displaystyle {\begin{aligned}\langle X,Y,Z\rangle =&XYZ\\&-\operatorname {E} Y\cdot XZ\\&-\operatorname {E} Z\cdot XY\\&-\operatorname {E} X\cdot YZ\\&+2(\operatorname {E} Y)(\operatorname {E} Z)\cdot X\\&+2(\operatorname {E} X)(\operatorname {E} Z)\cdot Y\\&+2(\operatorname {E} X)(\operatorname {E} Y)\cdot Z\\&-\operatorname {E} (XZ)\cdot Y\\&-\operatorname {E} (XY)\cdot Z\\&-\operatorname {E} (YZ)\cdot X\\&-\operatorname {E} (XYZ)\\&+2\operatorname {E} (XY)\operatorname {E} Z+2\operatorname {E} (XZ)\operatorname {E} Y+2\operatorname {E} (YZ)\operatorname {E} X\\&-6(\operatorname {E} X)(\operatorname {E} Y)(\operatorname {E} Z).\end{aligned}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/24d4b5c322230acf54aa138747ca3696c2b023d9)
Another notational convention
In the notation conventional among physicists, the Wick product is often denoted thus:
![{\displaystyle :X_{1},\dots ,X_{k}:\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/4c8f725347dcd6bd20c14fd6eb74ba1900225c0c)
and the angle-bracket notation
![{\displaystyle \langle X\rangle \,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/ab6889e5061d0a004dd0f74d4b75205dc1b75b85)
is used to denote the expected value of the random variable X.
Wick powers
The nth Wick power of a random variable X is the Wick product
![{\displaystyle X'^{n}=\langle X,\dots ,X\rangle \,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/34d9f8d8a9267b36d6dfb500201cf526b4791695)
with n factors.
The sequence of polynomials Pn such that
![{\displaystyle P_{n}(X)=\langle X,\dots ,X\rangle =X'^{n}\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/2d7b1e2f389c060a3222c8d2995a0a95a5789a71)
form an Appell sequence, i.e. they satisfy the identity
![{\displaystyle P_{n}'(x)=nP_{n-1}(x),\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/6212c21fcaf8552f47cf2de033fbcc6cfc1b48f9)
for n = 0, 1, 2, ... and P0(x) is a nonzero constant.
For example, it can be shown that if X is uniformly distributed on the interval [0, 1], then
![{\displaystyle X'^{n}=B_{n}(X)\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/e3ba5db8d675c82ea6fe2fa090b2cf3dc12cbfe0)
where Bn is the nth-degree Bernoulli polynomial. Similarly, if X is normally distributed with variance 1, then
![{\displaystyle X'^{n}=H_{n}(X)\,}](https://wikimedia.org/api/rest_v1/media/math/render/svg/aee4547083b0f60d844a54e8e573df12895ab107)
where Hn is the nth Hermite polynomial.
Binomial theorem
![{\displaystyle (aX+bY)^{'n}=\sum _{i=0}^{n}{n \choose i}a^{i}b^{n-i}X^{'i}Y^{'{n-i}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8197714a87400dac557bda3fee96079b324c9478)
Wick exponential
![{\displaystyle \langle \operatorname {exp} (aX)\rangle \ {\stackrel {\mathrm {def} }{=}}\ \sum _{i=0}^{\infty }{\frac {a^{i}}{i!}}X^{'i}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/974447b1270bca3ee7130b0011882e2dcac85d50)
References
- Wick Product Springer Encyclopedia of Mathematics
- Florin Avram and Murad Taqqu, (1987) "Noncentral Limit Theorems and Appell Polynomials", Annals of Probability, volume 15, number 2, pages 767—775, 1987.
- Hida, T. and Ikeda, N. (1967) "Analysis on Hilbert space with reproducing kernel arising from multiple Wiener integral". Proc. Fifth Berkeley Sympos. Math. Statist. and Probability (Berkeley, Calif., 1965/66). Vol. II: Contributions to Probability Theory, Part 1 pp. 117–143 Univ. California Press
- Wick, G. C. (1950) "The evaluation of the collision matrix". Physical Rev. 80 (2), 268–272.